Say Y here if you want to plug devices into your computer while
the system is running, and be able to use them quickly. In many
cases, the devices can likewise be unplugged at any time too.

One well known example of this is PCMCIA- or PC-cards, credit-card
size devices such as network cards, modems or hard drives which are
plugged into slots found on all modern laptop computers. Another
example, used on modern desktops as well as laptops, is USB.

Enable HOTPLUG and KMOD, and build a modular kernel. Get agent
software (at <http://linux-hotplug.sourceforge.net/>) and install it.
Then your kernel will automatically call out to a user mode "policy
agent" (/sbin/hotplug) to load modules and set up software needed
to use devices as you hotplug them.

source code:

drivers/pci/hotplug.c